According to Patent Document 1, in order to specify a common key used for encryption, data is transmitted to an apparatus at the transmission destination by appending a common key number to a non-encrypted portion of transmission data. Meanwhile, upon receipt of the data, the apparatus at the transmission destination decrypts the data by selecting the common key to be used for decryption from the common key number appended to the reception data. An apparatus authentication management system disclosed therein therefore enables communications between the transmission source and the transmission destination using the same common key, and the system is capable of preventing an event that communications between apparatuses are disabled in a case where the expiration date of the common key ends at different times.    Patent Document 1: JP-A-2003-101533